A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Within a lithographic apparatus, for example, it is desirable to ensure that contaminant levels remain below a particular level. In particular, within a lithographic apparatus using EUV radiation, contaminants containing carbon may reduce the lifetime of optical elements. This is because the carbon-containing molecules may be adsorbed onto an optical element and subsequently degrade to graphitic carbon when irradiated by EUV radiation. Such a carbon deposit decreases the reflectivity or transmissivity of the optical element.
It is therefore desirable to monitor the contaminant levels within the lithographic apparatus. Previously, it has been known to monitor contamination levels within lithography apparatus using residual gas analyzers. These are essentially mass spectrometers. However, there are some drawbacks in using residual gas analyzers for monitoring contaminant levels.
Firstly, in order to calculate a partial pressure of a contaminant, it is also necessary to provide a total pressure measurement of the gas within the lithographic apparatus. This increases the cost of the measurement system and the provision of two separate measurement devices increases the volume requirement for the measurement system, which is undesirable due to the space restrictions within a lithographic apparatus.
Furthermore, the provision of two measurement sensors means that more work is needed to calibrate the sensors and, in particular, that care must be taken to synchronize the calibration of the two sensors in order to avoid introducing additional possible errors. In addition, over the lifetime of the measurement system, the responses of the two sensors may diverge. Accordingly, repeated calibration of the measurement system may be needed.
Finally, when using a residual gas analyzer, it may be difficult to provide the required contaminant sensitivity. This is because, although the absolute sensitivity of the residual gas analyzers may be sufficient to measure a contaminant to a required partial pressure accuracy, it may only be possible in high vacuum due to a limit on the relative level of detection, namely the fraction of the total pressure that corresponds to the contaminant. Alternatively, it may only be possible using residual gas analyzers that are too expensive for practical use within a lithography apparatus and/or require a greater volume than is available within a practical lithography apparatus.